function CkUsername() {	
	if (($F('user_username').length)<6) {
		$('valid').innerHTML = "6 caractères minimum";
	} else {
		var url = '/api/CkUsername.php';
		var myAjax = new Ajax.Request(
			url, 
			{method: 'get',
			parameters: {UserName: $F('user_username')},
			onSuccess: processCkUsername,
			onFailure: function() { alert('none'); }
		});
	}
}
function processCkUsername(transport) {
	var response = transport.responseText;
	if(response == 'available') {
		$('valid').style.display="none";
	}
	else {
		$('valid').style.display="inline";
		$('valid').innerHTML = "Le pseudo <strong>"+ $F('user_username') +"</strong> est déjà utilisé.";
	}
}